WEBVTT 00:00:06.814 --> 00:00:14.045 The average 20 year old knows between 27,000 and 52,000 different words. 00:00:14.045 --> 00:00:20.053 By age 60, that number averages between 35,000 and 56,000. 00:00:20.053 --> 00:00:24.330 Spoken out loud, most of these words last less than a second. 00:00:24.330 --> 00:00:28.535 So with every word, the brain has a quick decision to make: 00:00:28.535 --> 00:00:32.235 which of those thousands of options matches the signal? 00:00:32.235 --> 00:00:36.345 About 98% of the time, the brain chooses the correct word. NOTE Paragraph 00:00:36.345 --> 00:00:37.475 But how? 00:00:37.475 --> 00:00:41.115 Speech comprehension is different from reading comprehension, 00:00:41.115 --> 00:00:44.375 but it’s similar to sign language comprehension— 00:00:44.375 --> 00:00:48.861 though spoken word recognition has been studied more than sign language. 00:00:48.861 --> 00:00:51.421 The key to our ability to understand speech 00:00:51.421 --> 00:00:54.691 is the brain’s role as a parallel processor, 00:00:54.691 --> 00:00:58.691 meaning that it can do multiple different things at the same time. 00:00:58.691 --> 00:01:01.301 Most theories assume that each word we know 00:01:01.301 --> 00:01:05.771 is represented by a separate processing unit that has just one job: 00:01:05.771 --> 00:01:10.931 to assess the likelihood of incoming speech matching that particular word. NOTE Paragraph 00:01:10.931 --> 00:01:15.139 In the context of the brain, the processing unit that represents a word 00:01:15.139 --> 00:01:19.796 is likely a pattern of firing activity across a group of neurons 00:01:19.796 --> 00:01:21.686 in the brain’s cortex. 00:01:21.686 --> 00:01:23.506 When we hear the beginning of a word, 00:01:23.506 --> 00:01:27.286 several thousand such units may become active, 00:01:27.286 --> 00:01:29.352 because with just the beginning of a word, 00:01:29.352 --> 00:01:31.532 there are many possible matches. 00:01:31.532 --> 00:01:35.535 Then, as the word goes on, more and more units register 00:01:35.535 --> 00:01:40.666 that some vital piece of information is missing and lose activity. 00:01:40.666 --> 00:01:43.126 Possibly well before the end of the word, 00:01:43.126 --> 00:01:48.090 just one firing pattern remains active, corresponding to one word. 00:01:48.090 --> 00:01:50.828 This is called the "recognition point." 00:01:50.828 --> 00:01:53.648 In the process of honing in on one word, 00:01:53.648 --> 00:01:56.718 the active units suppress the activity of others, 00:01:56.718 --> 00:01:58.838 saving vital milliseconds. 00:01:58.838 --> 00:02:03.635 Most people can comprehend up to about 8 syllables per second. NOTE Paragraph 00:02:03.635 --> 00:02:06.965 Yet, the goal is not only to recognize the word, 00:02:06.965 --> 00:02:10.415 but also to access its stored meaning. 00:02:10.415 --> 00:02:14.195 The brain accesses many possible meanings at the same time, 00:02:14.195 --> 00:02:16.875 before the word has been fully identified. 00:02:16.875 --> 00:02:22.018 We know this from studies which show that even upon hearing a word fragment— 00:02:22.018 --> 00:02:23.298 like "cap"— 00:02:23.298 --> 00:02:26.798 listeners will start to register multiple possible meanings, 00:02:26.798 --> 00:02:31.970 like captain or capital, before the full word emerges. NOTE Paragraph 00:02:31.970 --> 00:02:35.120 This suggests that every time we hear a word 00:02:35.120 --> 00:02:38.480 there’s a brief explosion of meanings in our minds, 00:02:38.480 --> 00:02:43.291 and by the recognition point the brain has settled on one interpretation. 00:02:43.291 --> 00:02:46.221 The recognition process moves more rapidly 00:02:46.221 --> 00:02:50.821 with a sentence that gives us context than in a random string of words. 00:02:50.821 --> 00:02:55.009 Context also helps guide us towards the intended meaning of words 00:02:55.009 --> 00:02:59.009 with multiple interpretations, like "bat," or "crane," 00:02:59.009 --> 00:03:03.009 or in cases of homophones like "no" or "know." 00:03:03.009 --> 00:03:07.393 For multilingual people, the language they are listening to is another cue, 00:03:07.393 --> 00:03:12.706 used to eliminate potential words that don’t match the language context. NOTE Paragraph 00:03:12.706 --> 00:03:16.706 So, what about adding completely new words to this system? 00:03:16.706 --> 00:03:20.706 Even as adults, we may come across a new word every few days. 00:03:20.706 --> 00:03:25.109 But if every word is represented as a fine-tuned pattern of activity 00:03:25.109 --> 00:03:27.439 distributed over many neurons, 00:03:27.439 --> 00:03:31.992 how do we prevent new words from overwriting old ones? 00:03:31.992 --> 00:03:34.322 We think that to avoid this problem, 00:03:34.322 --> 00:03:39.085 new words are initially stored in a part of the brain called the hippocampus, 00:03:39.085 --> 00:03:42.693 well away from the main store of words in the cortex, 00:03:42.693 --> 00:03:46.063 so they don’t share neurons with others words. NOTE Paragraph 00:03:46.063 --> 00:03:49.073 Then, over multiple nights of sleep, 00:03:49.073 --> 00:03:54.470 the new words gradually transfer over and interweave with old ones. 00:03:54.470 --> 00:03:57.990 Researchers think this gradual acquisition process 00:03:57.990 --> 00:04:01.354 helps avoid disrupting existing words. NOTE Paragraph 00:04:01.354 --> 00:04:02.774 So in the daytime, 00:04:02.774 --> 00:04:07.304 unconscious activity generates explosions of meaning as we chat away. 00:04:07.304 --> 00:04:12.305 At night, we rest, but our brains are busy integrating new knowledge 00:04:12.305 --> 00:04:14.125 into the word network. 00:04:14.125 --> 00:04:17.655 When we wake up, this process ensures that we’re ready 00:04:17.596 --> 00:04:20.696 for the ever-changing world of language.